A fundamental acquisition procedure in a cellular wireless system is cell search, which is performed by a terminal for obtaining time synchronization and frequency synchronization to a cell in the network and detecting cell identity of the cell. More precisely, there are two kinds of cell search: initial cell search and neighbor cell search. Initial cell search is performed when the terminal is powered on and comprises obtaining synchronization, cell identification, and receiving of a broadcast channel containing necessary system information for the terminal to access the cell. Neighbor cell search is performed by the terminal once it is connected to the network in search for candidate cells for handover. Hence, it typically does not involve reception of a broadcast channel. Rather once a cell has been detected, the terminal may feedback measurements of the cell to the base station, as to aid handover decisions. In practice, neighbor cell search occurs much more frequently than initial cell search. Cell search is enabled by the reception of synchronization channels transmitted from a base station.
The above procedures are incorporated in a 3GPP LTE system, wherein a mobile terminal (i.e., a UE) receives and transmits data on one cell at a time. However, in LTE-HW Advanced so called carrier aggregation is introduced, for which the UE is able to simultaneously receive (or transmit) data on multiple downlink (or uplink) carriers. In this way, higher data rates can be achieved. According to the LTE-Advanced standard, a Primary cell (PCell) and a Secondary cell (SCell) are defined. In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC), while in the uplink it is the Uplink Primary Component Carrier (UL PCC). Secondary Cells (SCells) can be configured to form together with the PCell a set of serving cells. In the downlink, the carrier corresponding to a SCell is a Downlink Secondary Component Carrier (DL SCC) while in the uplink it is an Uplink Secondary Component Carrier (UL SCC). The notion of cell is not necessarily related to a geographical area. Rather, it is a logical description and several cells can be transmitted from the same physical location and base station (eNodeB). The number of serving cells that can be configured depends on the aggregation capability of the UE. When adding a new SCell, dedicated RRC signaling is used for sending all required system information of the SCell, i.e., while in connected mode, UEs need not acquire broadcast system information directly from the SCells.
A configured cell may be activated or deactivated. Typically, the UE maintains communication over its PCell and when the need for higher data rates occurs, the eNodeB may activate one or several configured SCells to establish more bandwidth resources in addition to the PCell. Once the UE has no need for high data rates, the eNodeB could deactivate one or several of its SCells, in order to reduce the power consumption in the UE. When a SCell is configured, the eNodeB sends related information concerning the SCell to the UE on channels of the PCell. Such information includes the carrier frequency of the SCell and its cell identity. In order for the UE to start receiving and transmitting on the SCell, synchronization to the DL SCC of the SCell still needs to be acquired.
Cell search is regarded as a procedure demanding much complexity and power in the UE, since finding synchronization requires correlators (i.e., matched filters) performing complex valued multiplications (due to matching the received signal to a replica signal). It is therefore important to have low-complex receiver implementations of the cell searcher. It should also function at very low Signal-to-Interference-plus-Noise-Ratio (SINR) which may necessitate accumulation of correlation values over many radio frames. Low SINRs may not only be the case at distances far from the transmitter. High interference situations may also be common in heterogeneous network deployments, i.e., when small low-power cells (pico cells, femto cells, Home eNodeBs etc.) are deployed at the same carrier frequency as a high-power macro cell and in its coverage region. In that case, a UE that is connected to a pico cell may, even though the path loss is smaller to the pico cell than the macro cell, experience large interference from the macro cell, due to its higher transmit power. The UE may therefore not necessarily be connected to the cell with the largest received power. This implies that the SINRs could be far less than 0 dB. These severe situations could be handled for the data channels by coordinated scheduling between the macro and pico cell. However, there are no means for interference coordination of the synchronization channels, which makes synchronization a problem.